For example, Japanese Patent Application Laid-Open Publication No. 2013-26708 (Patent Document 1) discloses a method which makes lower switches automatically set a link aggregation group (hereinafter, abbreviated as LAG) in a network system having a plurality of upper switches and a plurality of lower switches. Concretely, each upper switch transmits a control frame containing the number of lower switches connected to itself to each lower switch, and each lower switch sets the LAG to ports which have received the control frames containing the largest number of lower switches among the numbers of lower switches contained in the control frames.